US2273762A - Incandescible cathode - Google Patents

Description

Feb.17,1942. E. H. REERINK ETAL 2,273,762

INCANDESCIBLGATHODE Filed April l5. 1935 Patented Feb. 17, 1942v ITED STATES PATENT OFFICE 2,273,762 INCANDESCIBLE .cA'rnoDE Conn., as trustee Application April 15, 1935, Serial No. 16,486

Inl Germany April 17, 1934 2 Claims.

The present invention relates to activated in`- candescible rcathodes of high electron-emitting capacity and which are'particularly adapted `v`for use in large capacity ion discharge tubes.

The term ion discharge tube as used herein, is to be understood to mean a discharge tube having a gaseous filling which may be either a gas or a vapor, or a mixture of gases and/or vapors, and in which the discharge ycurrent is carried primarily by the ionized gaseous iilling.

When such an ion discharge tube is destined for large capacities, its cathode must possessA a large electron-emitting capacity. This is obtained by providing a coating of ay highly elec-4 tron-emissive substance on the cathode, bywincreasing the effective electron-emitting surface, and by providing an ample supply of such highly emissive substances. To obtain this, various constructions have been suggested, for example by using recessed or channeled cathode bodies holding a large supply of high emissive substance.'l

The present invention provides a novel cone struction for a cathode of high electron-emitting capacity, which is comparatively simple and inexpensive, and which gives highly satisfactory allround results.

According to the invention, the desired results are obtained by Winding around a metal core, preferably made of refractory material, one or more thin metal wires to form around the core a structure in which a comparatively large mass and large area of highly electron-emissive substance surrounds the corev and is securely retained by. the turns and/or layers of the thin wire. Thereby a very large emitting surface can The` purpose of` this prior art construction zwas to provide a structure in `which theelectronemitting substance firmly adheres. 'I'he thin helical wire was comparatively vclosely Wound and the electron-emitting surfacecomparedto that `of a .simple `coated wire was not materially increased, nor was the quantity of electron-emit-y ting substance that could be held on thecathode.

According tothe present invention the thin wire is coiled before being Wound on the-,core and/or is wound in a plurality, of layers, or a plurality of wires are 4wound on top ofeach other.

In any case there are formed slots or recesses (Cl. Z50-27.5)

Vwithin the coils of the wire and/or between adjacent'turns and/or between superimposed layers adapted to hold a large amount of highly electron-emitting substance, whereas the wire affords for the secure suppOrt and adherence to the core of this substance.

The capillary slots or interspaces produced in this construction and lled with electron-emissive substance are particularly important in the case of iondischarge tubes, as in such tubes the gas or vapor ions neutralize `the negative space Charge of the incandescent cathode and thus the emitting particles located even at considerable depths of the slots and recesses can participate in the electron emission. Furthermore, the amount of emitting substance can be greatly increased andan ample supply thereof is provided. Thus cathodes constructed according to the invention can be built for very high current-carrying capacities.

The core Wire itself rmay and preferably does form a helix, Whereas the thin helically-wound wire surrounding the core can be applied `thereto to form a plurality of layers which `may be spaced in various manners. Instead of winding the thin Wire in several layers around the core it may be wound'into a closely-Wound helix, and this helix lWound in one or more layers around the core.

The forming or coiling of the surrounding thin wire, 4its application to the core, and the provision of the highly electron-emitting substance thereupon, can be achieved in various manners, examples of Which will be given in the detailed description of the invention.

In order that the invention may be clearly understood and readily carried into effect, several embodiments thereof will be described more fully with reference to the accompanying drawing, in which:

Figure l is a perspective view of one type of cathode made in accordance with the invention:

Fig. 2 is a greatly enlarged cross-sectional view of a cathode of the type shown in Figure 1, the section being taken between two successive turns of the coiled-coil wire;

Fig. 3 is an enlarged cross-sectional view of a cathode Ymade according to a somewhat diiferent embodiment of the invention; the section being takenv through the ,coiled-coil.

'.Fig, 4 is an enlarged view of part of a core wire with two layers of wire surrounding same as helices.` 1

Fig. 5 is a side View of a cathode structure constructed in accordance with Fig. 4;

Fig. 6 is an venlarged cross section through a portion of the cathode of Fig.` 5;

,Fg. 7 is an ion discharge tube using a ,cathode in accordance with the invention.

Referring to Figure 1, a core Wire I, preferably consisting of a refractory metal, for example of tungsten, forms a helix, as indicated by the dotted lines. Helically wound around the core wire I to form turns II (see Fig. 1) is a thin wire which itself is coiled to form turns I2, shown enlarged in Fig. 2; the thin wire thus forming a coiled-coil 2. The material of the coiled-coil 2 is preferably of refractory metal, for example tungsten, and as shown in Fig. 1, interspaces I are provided between the successive secondary turns II-I I.

The coiled-coil 2, as far as its outer shape is concerned, approximates a solid wire, and also performs all of the functions of a solid wire. In addition, however, the coiled-coil 2, forms a hollow body, the hollow of which corresponds to the inside diameter of the primary turns I2-I2, and which also has a recessed surface. This permits the provision of a large amount of electron-emissive substance on and within the coiledcoil 2. This, together with the interspaces I0 formed between the successive turns II, permits thus the application of a large amount of electron-emissive substance on the cathode, and also provides a very large emitting surface.

Even if the hollows of the coiled-coil 2 are not filled with electron-emissive substance, but the turns I2 are only coated on their surface, there is a considerable increase of the emitting surface, compared to the case when a simple solid wire is used to surround the core.

To insure a iirm hold on the electron-emissive substance as Well as a rigid positioning of the coiled-'coil 2 on the core, the diameter of the turns I2 should not exceed 300 microns. The diameter of the wire used and the pitch of the turns II and I2 depend upon the diameter of the core wire.

As shown in Fig, 2, an electron-emissive substance 3, for example, an earth metal oxide, as bariumv oxide, is .provided on and around the coiled-coil 2 so as to ll the spaces within and around the primary turns I2 and the spaces between the secondary turns I l. As a result thereof the active electron-emitting material forms a heavy annular layer around the core bounded by the dotted line I3, within which lies the coiledcoil 2; the latter serving to mechanically secure the electron-emissive substance 3 to the core wire I.

Instead of filling all of the vapertures and recesses of coiled-coil 2, a construction may be used as shown in Fig. 3 (where the section is taken through one of the turns II in which the Wire forming the coiled-coil 2' is provided with a uniform andcomparatively thin layer 3' of an electron-emissive substance.

Both of the constructions shown in Fig. 2 and Fig. 3 can be advantageously used; the construction vof Fig. 3 providing for a better adherence of the electron-emissive material, whereas the construction of Fig. 2 permits the application of a large amount of electron-emissive substance. In the latter case if some of the cathode-emitting substance falls off after prolonged use, sufficient emissive substance vremains'to activate the entire surface.` In either case the cathode is capable of copiously emitting electrons, and therefore a very long life of the cathode is insured.v`

' It should be noted that cathodes wound according to the linvention give an electron emission up to about four times as high as a cathode having the same outer dimensions and used under the same conditions, but having a core surrounded with a solid helix. u Y f To make a cathode according to the invention the core wire I may be given its helical shape prior to its being surrounded by the coiled-coil 2,

,in which case the coiled-coil 2 is separately formed. Thereby the thin wire is rst subjected to primary coiling and thereafter helically wound around a mandrel having the same diameter as the core wire I. After removing this mandrel, the coiled-coil 2 is slipped over the core wire I,

Again, the core wire I may be used in its straight condition as the mandrel upon which the coiled-coil 2 is formed, after whichY the core wire I with the coiled-coil 2 surrounding same, is subjected to coiling.

It is of course obvious that for the manufacture` of very large cathodes, this operation may be repeated once of several times, winding the secondary coil obtained in the former process on a new mandrel, to form a tertiary coil and so on.

The coiled-coil 2 may be formed by known methods, for example. by helically winding a tungsten wire on a mandrel, for instance on a molybdenum wire, which can then be easily relmoved, without damaging the tungsten wire, by

dissolving the molybdenum in a bath containing a mixture of concentrated sulphuric acid and strong nitric acid. The removal of the molybdenum mandrel may also take place at later stages of the manufacture, even as the last operation prior t0 the application of the electronemissive substance to the cathode.

The layer of emissive substance may be formed, for instance, by immersing the core I, with the coiled-coil 2 applied thereto, into a bath of molten barium hydroxide [Ba(OI-I2)l which, after the cathode has rbeen mounted into the discharge tube, is converted in known manner by heat into barium oxide.

Instead of, using barium hydroxide as thev starting material, barium carbonate may be used or again, a paste consisting of barium and strontium carbonates, and of a binding agent, which paste can be applied to the cathode by means of a brush, .and then treated by heat in known tube for 8.5 amps.maximum current value, a

suitable construction in accordance with the invention is the following:

The coiled-coil 2 is formed of a tungsten wire having a diameter of '75 microns.Y The primary coiling of this wire takes place with a pitch of 200 microns on a molybdenum mandrel having a diameter of microns. The secondary coiling of coiled-coil 2l then takes place with apitch of 400 microns on a tungsten wire, which may form the permanent core wire and which has a diameter of 450 microns.

The tungsten core wire itself is 4 then coiled with a pitch of 5 mm. on a mandrel having a diameter of 3 mm. The latter mandrel can then be pulled out, whereas the molybdenum core used for the primary coiling can be removed by a cathode is, for example, one in which the coiled-l coil is formed of a tungsten Wirehaving a suggest themselves.

micron diameter. This tungsten wire is provided in its primary coil by winding it with a pitch ofl a diameter of 8.5 mm.

Such a cathode structure with its core having eight turns gives a surface area of about 100 sq. cm. and is adapted to emit a maximum current of about 90 amps.

Instead of using a coiled-coil wire to surround the core, which requires a somewhat expensive process; the increased holding capacity of the cathode for the electron-emitting substance and the large effective surface area of the cathode may also be obtained by helically Winding around the core wire a plurality of spaced layers of simple wire.

Such a construction is shown in Fig. 4 in which two wires 4 and 5, made for example, of tungsten, and having a diameter of about .l to .5 Y

mm., are helically Wound in two superimposed layers around the core I having a diameter of from about .5 to 2 mm. The wires 4 and 5 are preferably wound in opposite directions on the core I to form reverse helices. `The core l with the wires 4 and 5 thereon form thus a cathode as k shown in Fig. 5.

The advantage in winding successive layers in opposite directions is due to the fact that the oppositely-directed stresses in the coils provide for a better grip upon the core wire. Such arrangement of the windings also further increases the structures holding capacity for the electronemissive substance, and prevents the turns of a layer from falling between turns of the preceding layer, as would be the case if successive layers were wound in the same direction.

The layers are preferably wound with a pitch greater than is the diameter of the wire, whereby the holding capacity of the cathode is greatly increased. As a rule it is advantageous to make the pitch at least twice as large as is the diameter of the Wire.

Fig. 6 is a greatly enlarged sectional view of a portion of the cathode shown in Fig. 5, whereby it will be noted that the electron-emissive substance 3 forms around the core a massive layer Within which are embedded the wires 4 and 5 Vthus the electron-emitting substance is mechanically well secured to the core and in intimate electrical contact therewith, whereas a very large amount of electron-emitting substance is provided and a large active surface obtained.

Fig. 7 is an ion discharge tube yieldingA a double phase rectied current of 6 amps. A discharge vessel I4 contains an incandescent cathode I 5 according to the invention. The electrodes are connected to conductors I1 leading to four prongs I9 of a cap I8. The discharge vessel contains a gas filling of 0.7 mm. xenon.

It should be noted that various modifications of any of the constructions shown in Figs. 1 to 3 or the constructions shown in Figs. 4 to 6 can The essential feature of the invention is a cathode having a metal wire core on which one or more thin metal wires are so arranged helically that they surround the core and at the same time form around the core several metal layers with interspaces and recesses which are filled out with a highly electron-emissive substance, and whereby the metal layers provide for the mechanical strength and good adherence of the electron-emissive substance to the core, and also for the provision on the cathode of a large mass of electron-emissive substance having a large effective surface. It should be also understood that even if a single coiled-coil is used, as for example as shown in Fig. 3, there are actually two metal layers.

It should be noted that in past constructions,

rwhen a thin wire coated with a highly-emissive necessary for the good adherenceof the coating to use a material like nickel, to which the highly emissive substance Well adhered. In general, however, the use of nickel introduces some disadvantages, especially due to its lower melting point as compared with highly refractory metals, such as molybdenum, and especially tungsten. In the constructions according to the invention, as a rule such refractory metals can be used both for the core `wire and for the thin wires surrounding same, because the construction itself provides a good adherence of the electron-emissive substance Within the recesses, hollows, and slots formed by the layer of the thin wire. In some cases, however, especially where a low cathode temperature may be used and there is no danger of the nickel fusing, the core wire, as Well as the wire surrounding same, may be made of a refractory metal, for example tungsten, which is provided with a nickel plating.

Thereby, depending upon the use and temperatures prevailing and the mechanical rigidity of the structure, one or more parts of the-cathode structure may be provided with a nickel layer, whereas other parts may lat the same time not be provided with nickel layers. Similarly, some of the parts, instead of being of a refractory metal coated with nickel, may be entirely of nickel.

In the claims by individual parts of the cathode is meant the core wire and the helix or the several helices surrounding the core.

While we have described our invention on hand of specific examples and in specific applications, we do not wish to be limited thereto, but desire the appended claims to be construed as broadly as permissible in View of the prior art.

What we claim is:

l. An incandescible cathode comprising a metal core wire, a hollow helix of thin metal wire, said helix being helically wound on said core wire and in contacting relationship therewith, and a highly electron-emissive substance carried by said helically-wound helix and extending into the hollow thereof.

2. An incandescible cathode comprising a metal core wire, a hollow helix of thin metal wire, said helix being helically wound on said core and in contacting relationship therewith, and a highly electron-emissive substance on all surface-portions of said wire except the portions in contact With the core wire.

EN GBERT HARMEN REERINK. PAUL CHRISTIAAN VAN DER WILLIGEN.

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